1. Field of the Invention
The present invention relates to a method for producing a single crystal grown perpendicularly at a desired position and a needle-like single crystal thereby produced, which is useful as a probe pin for measuring electrical characteristics of semiconductor integrated circuits, or as a probe for a microvacuum device or an electron gun or for scanning probe microscopes including a scanning tunnel microscope and an atomic force microscope.
2. Discussion of Background
Firstly, to facilitate the understanding of the present invention, a method for forming a needle-like crystal at a desired position on a substrate will be described, which constitutes a background technology of the present invention. This method is the one disclosed by R. S. Wagner and W. C. Ellis in Appl. Phys. Letters 4 (1964) 89. FIG. 1 is drawings to illustrate such a method for forming a needle-like crystal. As shown in FIG. 1(a), layers 2 of Au are placed at predetermined positions on a silicon single crystal substrate 1 with the surface being a (111) face. Then, this assembly is heated in a gas atmosphere containing silicon such as SiH.sub.4 or SiCl.sub.4 at a temperature higher than the melting point of a Si--Au alloy. A Si--Au alloy has a low melting point, and droplets of this alloy will accordingly be formed at the positions where the layers 2 of Au are placed. Here, silicon will be taken in from the atmosphere by the thermal decomposition of the gas. Since the liquid-state material takes in silicon atoms more readily than other solid state materials, the silicon content in the droplets of the Si--Au alloy will gradually become excessive. This excess silicon will undergo epitaxial growth on the silicon substrate 1, whereby as shown in FIG. 1(b), needle-like crystals 3 will grow along a &lt;111 &gt; axial direction while having Si--Au alloy droplets 5 at the top. Further, each needle like crystal 3 is a single crystal and thus has the same crystal orientation as that of the substrate 1. Further, the diameter of the needle-like crystal 3 is substantially the same as the diameter of the droplet. The foregoing mechanism for crystal growth is called VLS (vapor-liquid-solid) growth and will be hereinafter referred to simply as VLS growth.
This method is applied not only to the growth of a silicon single crystal but also to the growth of other single crystals. For example, a LaB.sub.6 single crystal has a high melting point at a level of 2,530.degree. C., a high evaporation rate and a high reactivity and therefore is not suitable for a molten liquid crystal growth. From such a viewpoint, VLS growth capable of conducting crystal growth at a low temperature has been attempted for a LaB.sub.6 single crystal (Journal of Crystal Growth 51 (1981) 190-194). Using a LaB.sub.6 single crystal substrate as the substrate, it is possible to form columnar crystals of LaB.sub.6 by patterning Au dots on the substrate. However, it is difficult to accurately form one columnar single crystal at one Au dot pattern, and the aspect ratio (thickness of Au/diameter of the pattern) must be at least 0.05. Even then, it is unavoidable that fine crystals will form around the central needle-like crystal (Journal of the Less-common Metals, 117 (1986) 97-103).
There are the following problems when single crystals are to be formed by the above described VLS growth method.
(1) When the aspect ratio (thickness/diameter) of a pattern (e.g. a spot-like pattern) is small, it is difficult to form one single crystal at one pattern, and a plurality of needle-like crystals or a bunch of fine needle-like crystals will be formed which are undesirable for forming a single crystal at a desired position on a substrate. (2) The positions of single crystals formed are likely to be dislocated from the initial positions of the corresponding patterns, and in an extreme case, the adjacent Au patterns are likely to contact each other during the formation of the Au--Si alloy, whereby a fusion phenomenon takes place where a plurality of patterns fuse to one another to form one pattern. (3) Needle-like single crystals grown corresponding to the individual patterns undergo kinking (bending) or branching (separation) during the growth.